Features of structure formation processes in AA2024 alloy joints formed by the friction stir welding with bobbin tool

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 2 2021 Fig. 9. Distribution of microhardness and grain size across the width of cross-sectional sample of the joint rier effect. But the most significant strengthening effect, in this case, maybe the formation of a supersatu - rated solid solution due to the dissolution of secondary phase particles and the enrichment of the aluminum matrix with alloying elements [10]. This is confirmed by the scattering of measured microhardness values, which exceed the error ranges due to the indenter hitting the particles of secondary phases. Conclusions Conditions of the welded joint formation in the BFSW process are determined by the heat input into the welded material, its fragmentation, and plastic flow along the tool contour, which depend on the ratio between the rotation and travel speeds of the welding tool. The adjacent mechanisms of the joint formation are based on the combination of equally important processes of the adhesive interaction in the “tool-material” system and the metal extrusion into the area behind the welding tool. In combination with heat dissipation conditions and the configuration of the “tool-material” system, this leads to the extrusion of material from the welded joint and its deconsolidation. As a result, extended defects are formed, with the characteristic defect forming at the end of the welded joint regardless of the welding mode. The increase in the tool travel speed allows the specific heat input to be reduced. However, during welding of long joints, the amount of heat released in the joint increases due to the characteristic heat dissipation conditions. All this leads to a change in the conditions of adhesive interaction and extrusion processes, which leads either to the growth of already existing defects or to the formation of new ones. Taking into account the complexity of mechanisms of welded joint structure formation by the BFSWmethod, the defect-free welded joint production supposes mandatory use of various non-destructive control methods in combination with adaptive regulation of technological parameters directly in the welding process. References 1. Threadgill P.L., Ahmed M.M.Z., Martin J.P., Perrett J.G., Wynne B.P. The use of bobbin tools for friction stir welding of aluminium alloys. Materials Science Forum , 2010, vol. 638–642, pp. 1179–1184. DOI: 10.4028/www. scientific.net/MSF.638-642.1179.